In high intensity arc discharge lamps, the radiated light output is derived from a plasma arc discharge within an arc tube. One form of high intensity discharge lamp that is currently and conventionally employed is the sodium iodide lamp. In such lamps the arc discharge tube includes sodium iodide which is vaporized and dissociated in the plasma arc during lamp operation. However, in the vicinity of the arc tube walls, where the temperature is cooler, sodium remains chemically bound to the iodine limiting the presence of free sodium which absorbs some of the light radiation from the arc discharge.
The self-absorption characteristics of cooler sodium atoms distributed preferentially near the cooler arc tube walls act to limit lamp efficacy. In particular, sodium D-line radiation produced within the hot central plasma region of the arc tube would be readily absorbed by the cooler sodium atoms which would be present near the arc tube walls.
While the use of sodium iodide in the lamp lessens the presence of free sodium near the cooler arc tube walls, the sodium to iodine ratio in this area remains greater than unity. With its smaller atomic mass, sodium diffuses to the arc tube walls more rapidly than iodine. Thus, lamp efficacy is still limited by the presence of free sodium near the arc tube walls.
The high pressure sodium iodide arc lamp requires the use of a buffer gas to limit the transport of energy from the arc discharge to the arc tube walls via chemical reaction. Mercury is conventionally employed as the buffer gas at a high pressure. However, high pressure mercury broadens the sodium D-line radiation toward the red and can tie-up iodine by forming mercury iodide, resulting in more free sodium near the arc tube walls. Copending application Serial No. (676,367), assigned to the assignee of the present invention, discloses xenon buffer gas for improving the efficacy of the high pressure sodium iodide arc lamp. However, even with xenon as the buffer gas, the sodium to iodine ratio in the vicinity of the arc tube walls remains greater than unity (i.e. some free sodium remains) during lamp operation.